A Tee coupler is a junction between three waveguides arranged in the form of a T, the three waveguides each comprising an end forming an input or output port of the coupler. The Tee junction can be of two different types, called a junction in the E-plane or in the H-plane, depending on the arrangement of the waveguides forming the three arms 10, 20, 30 of the T with respect to the electric field E and to the magnetic field H propagating in the waveguides. In a known manner, when an electromagnetic wave propagates in a rectangular waveguide, the electric field E extends along a direction perpendicular to the large sides of the waveguide and the magnetic field H extends along a direction parallel to the large sides of the waveguide.
The Tee coupler most commonly used for power splitters in waveguide technology is the H-plane Tee junction represented schematically in FIG. 1a. The waveguides have rectangular cross-section, each waveguide being delimited by a peripheral metallic wall consisting of two large sides, of two small sides and comprising an input or output port. The three input and output waveguides 10, 20 and 30 are mounted flat on their large side and extend in one and the same plane XY, the input waveguide 30 being perpendicular to the two lateral output waveguides 10 and 20. The junction is said to be in the H-plane since the output ports 11, 21 of the two lateral waveguides 10 and 20, which form the horizontal bar of a T, are oriented in the same plane XY as the H-field established in the input port 31 of the input waveguide 30.
The Tee junction in the H-plane is frequently used in a waveguide splitting array to connect the two output ports 11, 21 to two radiating elements 12, 22, such as for example compact horns, the assembly forming a radiating array which can be used in a planar antenna. The radiating array represented in FIG. 1b comprises an H-plane Tee junction mounted parallel to the plane XZ and two radiating horns oriented along the axis Z and connected to the two output ports of the Tee junction. For bulkiness reasons, in particular for the low frequency bands, it may be desired that the splitting array be situated in the plane XY thereby making it possible to reduce the thickness of the splitting array along the direction Z. In this case, the radiating elements can be fed by the splitting array by way of an electromagnetic coupling slot 13, 23 as shown by FIG. 1c. This coupling technique is sensitive to the direction of propagation of the incident electromagnetic wave. If the two radiating elements 12, 22 are excited by electromagnetic waves propagating in opposite directions, then they radiate in phase opposition. The splitting array must then compensate for this difference of excitation phase. If this splitting array consists of a Tee junction in the H-plane, so that the radiating elements are excited in phase by one and the same feed source and radiate in a coherent manner, it is necessary to add a stub 14, consisting of a waveguide segment, having a length equal to a guided half-wavelength, on one of the two output ports 11 or 21. This waveguide segment 14 carries out a phase inversion of 180° which compensates for the phase difference due to the excitation by an electromagnetic slot. This additional waveguide segment increases the distance between two radiating elements, as shown by the example of FIG. 1c in which the radiating array comprises an H-plane Tee junction oriented parallel to the plane XY and two radiating elements of horn type oriented along the direction Z. Moreover, the power splitter thus formed is dissymmetric, this being prejudicial to the passband performance of the radiating array.
To excite the radiating elements in phase with a symmetric and compact splitting array, it is then necessary to have a Tee coupler in the E-plane, as shown by FIGS. 2a and 2b. The Tee coupler in the E-plane represented schematically in FIG. 2a makes it possible to excite two radiating elements in phase, without requiring an additional waveguide segment. In this Tee junction in the E-plane, the two lateral waveguides 10 and 20 are mounted flat on their large side one behind the other along one and the same direction X of the plane XY and the input waveguide 30 is coupled perpendicularly to the two lateral waveguides 10 and 20 and extends along a direction Z perpendicular to the plane XY. The junction is said to be in the E-plane since the two output ports 11, 21 at the ends of the two lateral waveguides 10, 20 which form the transverse bar of a T are in the same plane XY as the field E established in the input port of the input waveguide 30. However, this known Tee junction is characterized by an input port 31 disposed along a direction Z normal to the plane XY formed by the large sides of the rectangular output guides. This disposition increases the bulkiness of the coupler in terms of height and the bulkiness of a power splitter and of a planar antenna comprising such a Tee coupler in the E-plane and radiating elements 12, 22 coupled to this power splitter by way of the respective coupling slots 13, 23.
As represented in FIG. 3, it is also possible to achieve a Tee coupler in the E-plane by mounting the input waveguide 30 and the two lateral output waveguides 10, 20 flat on two distinct stages overlaid one above the other, the large sides of all the waveguides 10, 20, 30 being parallel to the plane XY. In this case, the two lateral output waveguides are replaced with a single waveguide 40 linking the two output ports 11, 21. If the input waveguide 30 is disposed at the lower stage and the output waveguide 40 is situated at the upper stage, the coupling in the E-plane takes place by devising a slot 35 at the end of the input waveguide 30, in the upper wall, and a corresponding slot at the centre of the lower wall of the output waveguide 40 linking the two output ports. The coupling between the input port 31 and the output ports 11, 21 being in the E-plane, the two output ports 11, 21 can be connected to two radiating elements so that they radiate in phase coherence. It is thus not necessary to add a waveguide segment on one of the output ports, thereby improving the compactness of the power splitter obtained. However, to excite the lateral waveguides in a symmetric manner, it is necessary for the coupling slots to be made in the input waveguide in a dissymmetric manner. In particular, in FIG. 3, the coupling slot is disposed at the edge of the input waveguide and not at the centre. This therefore results, as in the case of a tee coupler in the H-plane, in a dissymmetry of the power splitter. This dissymmetry results in an unbalanced coupling between the output ports and also alters the passband of the antenna obtained. It is also detrimental to the compactness of the radiating array.